Sick euthyroid syndrome is common among infants and children following cardiac surgery using cardiopulmonary bypass. Recent data suggest that triiodothyronine (T3) supplementation after cardiac surgery may improve postoperative outcomes. We propose a randomized, double-blind, placebo controlled trial to evaluate the effect of T3 supplementation on the postoperative course and later neurodevelopmental outcome in neonates undergoing the Norwood procedure. T3 will be administered for 72 hours after cessation of bypass. Children in both treatment groups will receive conventional therapy. Based upon the literature and our own pilot study, we anticipate that T3 will have few adverse effects. Our first aim is to test the hypothesis that T3 supplementation, compared to placebo, will be associated with better early postoperative hemodynamic status and faster postoperative recovery. Our primary outcome measure will be a composite clinical outcome score, with values from 0-7, based upon the time until negative fluid balance is first achieved;time until sternal closure;time until first extubation;and the occurrence of death or use of ECMO. Secondary outcomes will include individual components of the clinical outcome score, as well as inotrope requirement over the initial 5 postoperative days, myocardial performance index, serum lactate accumulation following the termination of bypass;length of postoperative stay in the ICU and hospital;and incidence of serious adverse events. Our second aim is to test the hypothesis that T3 supplementation, compared to placebo, will be associated with superior developmental outcome. Our primary outcome variable will be score on the Psychodevelopment Index (PDI) of the Bayley Scales of Infant Development at age one year. Secondary outcomes will include the Mental Development Index (MDI) of the Bayley Scales, the MacArthur Communicative Developmental Index, and the Functional Status Il-Revised questionnaire. The structure of the study will allow us to explore the relationship of postoperative T3 levels to preoperative, intraoperative and early postoperative management strategies, including use of modifiers of thyroid function such as topical iodine, iodinated contrast, and dopamine. The trial will span three years from onset of enrollment until completion of follow-up. If T3 therapy is found to be advantageous to the Norwood population, it may benefit many other children with congenital heart disease who require surgery in early infancy. The information learned about the interrelationships of thyroid hormone status and other medical and surgical therapies in infants undergoing the Norwood procedure makes an important contribution to the overall management of these particularly complex and high-risk patients. This study takes advantage of the expertise and momentum accumulated in enrolling and managing infants undergoing the Norwood procedure in research protocols within the Pediatric Heart Network. (End of Abstract).